In stackleak_erase() we check skip_erasing() after accessing some fields
from current. As generating the address of current uses asm which
hazards with the static branch asm, this work is always performed, even
when the static branch is patched to jump to the return at the end of the
function.
This patch avoids this redundant work by moving the skip_erasing() check
earlier.
To avoid complicating initialization within stackleak_erase(), the body
of the function is split out into a __stackleak_erase() helper, with the
check left in a wrapper function. The __stackleak_erase() helper is
marked __always_inline to ensure that this is inlined into
stackleak_erase() and not instrumented.
Before this patch, on x86-64 w/ GCC 11.1.0 the start of the function is:
<stackleak_erase>:
65 48 8b 04 25 00 00 mov %gs:0x0,%rax
00 00
48 8b 48 20 mov 0x20(%rax),%rcx
48 8b 80 98 0a 00 00 mov 0xa98(%rax),%rax
66 90 xchg %ax,%ax <------------ static branch
48 89 c2 mov %rax,%rdx
48 29 ca sub %rcx,%rdx
48 81 fa ff 3f 00 00 cmp $0x3fff,%rdx
After this patch, on x86-64 w/ GCC 11.1.0 the start of the function is:
<stackleak_erase>:
0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) <--- static branch
65 48 8b 04 25 00 00 mov %gs:0x0,%rax
00 00
48 8b 48 20 mov 0x20(%rax),%rcx
48 8b 80 98 0a 00 00 mov 0xa98(%rax),%rax
48 89 c2 mov %rax,%rdx
48 29 ca sub %rcx,%rdx
48 81 fa ff 3f 00 00 cmp $0x3fff,%rdx
Before this patch, on arm64 w/ GCC 11.1.0 the start of the function is:
<stackleak_erase>:
d503245f bti c
d5384100 mrs x0, sp_el0
f9401003 ldr x3, [x0, #32]
f9451000 ldr x0, [x0, #2592]
d503201f nop <------------------------------- static branch
d503233f paciasp
cb030002 sub x2, x0, x3
d287ffe1 mov x1, #0x3fff
eb01005f cmp x2, x1
After this patch, on arm64 w/ GCC 11.1.0 the start of the function is:
<stackleak_erase>:
d503245f bti c
d503201f nop <------------------------------- static branch
d503233f paciasp
d5384100 mrs x0, sp_el0
f9401003 ldr x3, [x0, #32]
d287ffe1 mov x1, #0x3fff
f9451000 ldr x0, [x0, #2592]
cb030002 sub x2, x0, x3
eb01005f cmp x2, x1
While this may not be a huge win on its own, moving the static branch
will permit further optimization of the body of the function in
subsequent patches.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220427173128.2603085-3-mark.rutland@arm.com
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